With regards to the public's voting intentions - people would have to turn to the BNP/UKIP in huge numbers for any 'dent' to affect the main parties. Here's the thing...the vast majority of Brits are a) moderates who recognize that Britain has been a multicultural country for decades, b) more and more people have lived and worked side by side with immigrants, so to them, that's their 'normality' and c) they recognize that there are so many other factors which influences their votes.
If you vote BNP/UKIP, then you'd have to study their policies for economic growth, transport, the health service, business and personal tax structures, urban regeneration, further education etc etc.
The minor parties don't have the political synapses.